Conventional voice mail systems (also termed voice messaging systems) attach to the telephone network, often via a PBX, and are used to store messages from incoming calls when the intended recipient is absent or otherwise engaged. The intended recipient can then listen to their stored messages at some future time. A voice mail system is generally implemented either on special purpose computer hardware, or else on a standard computer workstation equipped with a suitable telephony interface. Such voice mail systems are well-known; one example is the DirectTalkMail system, available from IBM Corporation, which operates in conjunction with the DirectTalk/6000 voice processing system (also available from IBM). Other examples of voice mail systems are described in U.S. Pat. No. 5,136,648 and EPA 0588576.
Also known are electronic mail systems (email), which allow the transmission of text messages over a computer network. The most prominent such network over the last couple of years is the Internet, which provides a readily accessible, worldwide network for packet-based communications. Background information about the Internet and the World Wide Web can be found in "Spinning the Web" by Andrew Ford (International Thomson Publishing, London 1995) and "The World Wide Web Unleashed" by John December and Neil Randall (SAMS Publishing, Indianapolis 1994).
Although originally intended primarily for the transmission of computer data, more recently the Internet has been exploited to provide real-time telephony communications. The primary attraction of the Internet for telephony communications is the charging structure. Thus many Internet users have a dial-up connection to an access provider. This is normally over a local telephone line, so such users pay only local telephone charges when logged on. Some access providers charge a monthly description, whilst others charge on the basis of connection time (some may do both). However, there is generally no charge associated with actual data transfer over the network. As a result, the effective cost of an international call over the Internet may be no more than that of a local call of the same duration to the access provider. In addition, the fully digital nature of the Internet may potentially offer a richer functionality (eg in terms of conference calling) than conventional telephone networks. Internet telephones are surveyed in the article "Dial 1-800-Internet" in Byte Magazine, February 1996, p83-88, and in the article "Nattering On", in New Scientist, Mar. 2, 1996, p38-40.
The transmission of voice signals over a packet network is described for example in "Using Local Area Networks for Carrying Online Voice" by D Cohen, pages 13-21, in "Voice Transmission over an Ethernet Backbone" by P Ravasio, R Marcogliese, and R Novarese, pages 39-65, both in "Local Computer Networks" (edited by P Ravasio, G Hopkins, and N Naffah; North Holland, 1982) and also in GB 2283152. The basic principles of such a scheme are that a first terminal or workstation digitally samples a voice input signal at a regular rate (eg 8 kHz). A number of samples are then assembled into a data packet for transmission over the network to a second terminal, which then feeds the samples to a loudspeaker or equivalent device for playout, again at a constant 8 kHz rate. Voice transmission over the Internet is substantially similar to transmission over a LAN (which may indeed provide part of the Internet transmission path), but there tends to be less spare bandwidth available on the Internet. As a result, Internet telephones normally compress the voice signal at the transmitting end, and then decompress it at the receiving end.
There are several well-known problems with packet-based voice communications. Firstly, there is a transmission delay over the network, which is variable, depending particularly on the utilisation of the network by other nodes at any given time. Thus the arrival of packets at a destination node is both delayed and irregular. If the packets were played out in irregular fashion, this would have an extremely adverse effect on intelligibility of the voice signal. Therefore, packet voice transmissions utilise some degree of buffering at the reception end, to absorb such irregularities. Such buffering however increases the delay between the original voice signal and the audio output at the destination end. This delay may cause problems with echos, and more importantly, can render natural interactive two-way conversation difficult (in the same way that an excessive delay on a transatlantic conventional phone call can be highly intrusive). Yet further delays are introduced by the necessity to compress/decompress the voice signal.
Some Internet telephones which are currently commercially available provide a voice mail facility, whereby the calling party can leave a message if the called party does not answer. This is somewhat analogous to the conventional answerphone. Note that such systems are limited in that generally the called party is required to be logged onto the Internet to receive a voice message. However, many users with dial-up Internet access via a modem have to pay on a time basis for a telephone call into their Internet access provider, and moreover some Internet access providers charge for connections on a time basis. Therefore most users log off from the Internet when they are not using it to avoid incurring excessive charges. In such circumstances, this implies that they are no longer able to receive a voice message.
It would of course be possible for a user to transmit a voice message in the form of a normal email (data) message to another user who is not currently logged on. The message is then queued at the Internet access provider's site for delivery at next logon by the relevant user. However, generally this approach offers few benefits over simply sending an email message, rather than the voice message.
It is an object of the invention to provide improved voice mail facilities for the users of Internet telephones.